Such an antenna is disclosed in United Kingdom Patent Application No. GB2321785A. This known antenna has a pair of laterally opposed elongate antenna elements which extend between longitudinally spaced-apart positions on a solid dielectric core, the antenna elements being connected at respective first ends to a feed connection and at second ends to a balun sleeve. The antenna elements and sleeve are arranged so as to form at least two conductive paths extending around the core, wherein one of the two paths has an electrical length which is greater than that of the other path at an operating frequency of the antenna. This is achieved using forked antenna elements, wherein each element having a divided portion extending from a position between the top of the dielectric core and the rim of the balun sleeve, the divided portion of at least one of the antenna elements having branches of different electrical lengths. The balun sleeve is split in the sense that longitudinally extending slits are formed as breaks in the conductive material of the sleeve so as to provide isolation between the two sleeve parts, thus defining the two conducting paths. The balun slits are arranged to have an electrical length of about a quarter wavelength (.lambda./4) in the operating frequency band, the zero impedance point provided by the rim of the sleeve being transformed to a high impedance point between the divided elements, thereby isolating the sleeve parts from one another. As a result of the conductive paths having different electrical lengths, each conductive path resonates at a different frequency and so provides an antenna having a relatively wide bandwidth.
One problem associated with the above antenna is that it is difficult to incorporate slits of sufficient length within the sleeve to provide the quarter wavelength, especially if the sleeve is short. The L-shaped slits disclosed in GB2321785A can be difficult to manufacture and restrict the flow of currents in the sleeve.